An asymptotic approach for a semi‐Markovian inventory model of type (s,S)

In this study, we constructed a stochastic process (X(t)) that expresses a semi‐Markovian inventory model of type (s, S) and it is shown that this process is ergodic under some weak conditions. Moreover, we obtained exact and asymptotic expressions for the n^th order moments (n = 1,2,3, … ) of ergodic distribution of the process X(t), as S ? s → ∞ . Finally, we tested how close the obtained approximation formulas are to the exact expressions. Copyright © 2012 John Wiley & Sons, Ltd.     

Development of boundary transfer method in simulation of gas–solid turbulent flow of a riser

Computational fluid dynamics (CFD) is used to simulate the behavior of two phase gas solid in a fluid catalytic cracking (FCC) riser. Gas and particle phases are considered as separate fully interpenetrating continuous media within each control volume. Each phase described in terms of its own separate mass and momentum conservation equations. Simple k–epsilon (k_g–?_g) turbulence model is used for the gas phase and the solid phase is handled with the kinetic theory of granular flows. Source terms are used to account for the influence of hydrodynamic drag on the production, dissipation and exchange of turbulent kinetic energy between the phases. For the particles partial slip condition is considered at the wall.     

Inventory model of deteriorating items on non-periodic discrete-time domains

In this paper, we demonstrate how to model a discrete-time dynamic process on a non-periodic time domain with applications to operations research. We introduce a discrete-time model of inventory with deterioration on domains where time points may be unevenly spaced over a time interval. We formalize the average cost function composed of storage, depreciation and back-ordering costs. The optimal condition is given to locate the optimal point that minimizes the average cost function. Finally, we present simulations to demonstrate how a manager can use this model to make inventory decisions.     

Dynamic decision-making for an inventory system with time-varying demand

This article considers the problem of dynamic decision-making for time-varying demand products under trade credit. The article adopts a price, warranty length and time-dependent demand function to model the finite time horizon inventory. The objective of this study is to determine the optimal periodic selling price, warranty length and ordering quantity so that the total profit is maximized. We discuss the optimization properties and develop solution procedures based on dynamic programming techniques for solving the problem described. The numerical analyses show that dynamic decision-making is superior to fixed decision-making and an appropriate warranty policy can benefit the company. This study also discusses the effects of interest earned, interest charged and credit period on company's decisions and profits.     

Customer differentiated end-of-life inventory problem

This paper deals with the service parts end-of-life inventory problem in a circumstance that demands for service parts are differentiated. Customer differentiation might be due to criticality of the demand or based on various service contracts. In both cases, we model the problem as a finite horizon stochastic dynamic program and characterize the structure of the optimal inventory policy. We show that when customers are differentiated based on the demand criticality then the optimal structure consists of time and state dependent threshold levels for inventory rationing. In case of differentiation based on service contracts, we show that in addition to rationing thresholds we also need contract extension thresholds by which the system decides whether to offer an extension to an expiring contract or not. By numerical experiments in both cases, we identify the value of incorporating such decisions in service parts end-of-life inventory management with customer differentiation. Moreover, we show that these decisions not only result in cost efficiency but also decrease the risk of part obsolescence drastically.     

具有两类需求服务的随机库存系统控制策略研究

研究具有两类顾客排队需求服务的随机库存系统.系统采取(s,Q)补货策略且当库存水平下降到安全库存s时,到达的第二类顾客以概率P得到服务.首先,建立库存水平状态转移方程并通过递推算法求解获得库存水平稳态概率分布和系统稳态指标;接下来,构建库存成本函数;最后,采用数值试验的方法研究该库存系统的最优控制策略并考察系统参数的敏感性.     

JIT环境下工厂线边库存容量投资协调策略研究

在JIT环境下,制造商要求长期合作的供应商在其工厂周边租用仓库,而自己只维持较少的线边库存。本文以此为背景,假定制造商投资建设工厂线边库存的费用与其容量大小成正比,分别建立了供应链分散决策和集中决策模型,并提出了制造商工厂线边库存投资建设协调模型。研究结果表明,相比分散决策,供应链集中决策时制造商投资建设的最优工厂线边库存容量更大,从而有利于供应商和整个供应链,而不利于制造商。算例分析证明提出的基于不对称博弈的线边库存投资费用分担策略能够实现整个供应链利益的帕累托优化。     

能源回购补偿机制下基于无限阶段折扣准则的最优生产与库存策略

世界经济的快速发展和工业化进程的推进促使各国电力需求激增,电力供需矛盾为能源回购项目的发展提供了条件。为能够实现错峰用电和缓解能源需求的紧张,能源回购项目在每个阶段出现能源短缺时,将根据短缺的不同程度为限产(或停产)企业提供了金额不同的资金补偿。因此,在该能源回购补偿机制下,企业需要确定每个阶段是否参加能源回购项目及其相应的生产库存策略,来实现其期望折扣成本的最小化。本文研究了能源回购补偿机制下企业以最小化期望折扣成本为目标的无限阶段最优生产/库存策略。引入启动成本和多个能源需求状态的资金补偿水平后,在合理的假设条件下,证明了每个阶段生产商的最优生产/库存策略在高峰状态为(sⁱ,S)策略,在非高峰状态为(s⁰,S,A)策略。     

基于EDI交易的随机存储策略模型

利用数学建模的方法 ,研究了电子商务环境下的最优库存控制问题     

Experimental study of the effect of friction phenomena on actual and calculated inventory in a small-scale CFB riser

Accurate information concerning riser inventory in a fluidized bed is required in some applications such as the calcium looping process, because it is related to the CO₂ capture efficiency of the system. In a circulating fluidized bed (CFB), the riser inventory is normally calculated from the riser pressure drop; however, the friction and the acceleration phenomena may have a significant influence on the total riser pressure drop. Therefore, deviation may occur in the calculation from the actual mass. For this reason the magnitude of the friction and the acceleration pressure drop in the entire riser is studied in small-scale risers. Two series of studies were performed: the first one in a scaled cold model riser of the 10 kW_th facility, and the second one in the 10 kW_th fluidized bed riser under process conditions. The velocities were chosen to comply with the fluidization regimes suitable for the calcium looping process, namely, the turbulent and the fast. In cold-model experiments in a low-velocity turbulent fluidization regime, the actual weight (static pressure drop) of the particles is observed more than the weight calculated from a recorded pressure drop. This phenomenon is also repeated in pilot plant conditions. In the cold-model setup, the friction and acceleration pressure drop became apparent in the fast fluidization regime, and increased as the gas velocity rose. Within calcium looping conditions in the pilot plant operation, the static pressure drop was observed more than the recorded pressure drop. Therefore, as a conservative approach, the influence of friction pressure drop may be neglected while calculating the solid inventory of the riser. The concept of transit inventory is introduced as a fraction of total inventory, which lies in freefall zones of the CFB system. This fraction increases as gas velocity rises.